Until recently, the conventional methods of producing gears involved a hobbing or some other metal cutting operation in which metal from the gear blank was removed by successive and repeated passes of a tool through the blank, removing a small amount of metal on each pass. Recent developments in the extrusion art have demonstrated the possibility of forming gears by extrusion, a process which presents the advantage of forming the gear in a single pass between the die or tool and the gear blank. The nature of the extrusion process is such that the extrusion die must be provided with an axially extending draft or taper which permits the metal being extruded to expand slightly after it has passed through the narrowest portion of the extrusion passage of the die.
To form a die for extruding a metal gear, the die passage is formed as an internal gear. It is a relatively simple machining operation to form such an internal gear if its pitch circle is of uniform diameter over the entire axially extent of the gear. In an extrusion die, however, instead of having the pitch circle at all axial locations lie on a cylindrical surface, the pitch circles at all axial points must lie on a frusto conical surface in order to achieve the necessary draft or taper required by the extrusion process. The machining of an internal gear whose pitch circles lie on such a frusto conical surface is a complex machining operation, particularly where a die for extruding a helical gear is involved.
The present invention is especially directed to a method by which an extrusion die for forming helical gears or other articles of complexly modified generally circular cross sections may be accurately and efficiently formed by means of relatively simple shop procedures.